1. Field of the Invention
The present invention relates to a hydraulic control system for a 4 drive speed automatic transmission, and more specifically to a hydraulic pressure control system which can reduce speed-range shift shock produced when the 4 drive speed automatic transmission is shifted from the 4th speed range to the 2nd speed range.
2. Description of the Prior Art
In a conventional 4 drive speed automatic transmission of electronic control type as disclosed in U.S. Pat. No. 4,346,626, 4 drive speed ranges are shifted by energizing or deenergizing two solenoid valves, one communicating with a 2-3 shift valve, and the other communicating with a 1-2 shift valve and a 3-4 shift valve. In the prior-art transmission, however, there exists a need of providing independent valve bodies separately, for enabling a 1st to 3rd speed range shifting and a 4th speed range shifting, in order to realize a 4-speed automatic transmission, thus resulting in a drawback such that the transmission is costly.
Table 1 below shows the engagement relationship of clutches, brakes, etc. in the conventional 3-speed transmission; Table 2 below shows the same relationship in the conventional 4-speed transmission.
TABLE 1 ______________________________________ D 2 L R P.N 1st 2nd 3rd 1st 2nd 1st Rer -- ______________________________________ C.sub.1 o o o o o o -- -- C.sub.2 -- -- o -- -- -- o -- B.sub.1 -- o -- -- o -- -- -- B.sub.2 -- -- -- -- -- o o -- F o -- -- o -- -- -- -- ______________________________________
TABLE 2 ______________________________________ D 2 L R P.N 1st 2nd 3rd 4th 1st 2nd 1st Rer -- ______________________________________ C.sub.1 o o o o o o o -- -- C.sub.2 -- -- o o -- -- -- o -- B.sub.1 -- o -- -- -- o -- -- -- B.sub.2 -- -- -- -- -- -- o o -- F o -- -- -- o -- -- -- -- C.sub.3 -- -- -- o -- -- -- -- -- B.sub.3 o o o -- o o o o -- ______________________________________
In Tables 1 and 2 above, C.sub.1, C.sub.2 and C.sub.3 denote clutches; B.sub.1, B.sub.2 and B.sub.3 denote brakes; F denotes a one-way clutch; a circular mark (o) designates an engagement status; and a bar bark (-) designates a disengagement status.